1. Field of the Invention
The present invention relates to a robot emergency stop circuit for ensuring the safety of an operator when teaching a robot a job procedure through a teaching control device.
2. Background of the Related Art
In a known method of teaching behavior patterns to a robot and the like, in which a sequence of movements of the robot and an operating sequence are keyed to the robot's tools and the shape of a workpiece, an operator actually teaches these procedures to the robot through a teaching control device.
In such a method, to teach the robot an accurate positioning of the tools thereof, during the teaching session the operator must occasionally physically enter a robot working area and teach the robot while in the vicinity of the tools. In such a case, the operator carries a teaching control device provided with a robot emergency stop switch, called a deadman switch, to be able to stop the operation of the robot while in the working area thereof.
FIG. 3 shows a robot emergency stop circuit incorporating a traditional deadman switch, wherein all of the emergency stop pushbutton switches EMG1, EMG2, EMG3 and EMGT are connected in series with a deadman switch DM2 on the back of the teaching control device, through an alternating current power supply 1. In the diagram, four emergency stop pushbutton switches are shown, but in actual application, the number of emergency stop pushbutton switches corresponds to the number of pieces of external equipment to be controlled.
A servo amplifier (AMP) 2 supplies electric power to a servomotor which drives the robot, and includes a magnet switch operated by the alternating current power supply 1. Accordingly, if any of the switches, EMG1, EMG2, EMG3, EMGT or DM2 are turned OFF, then the electric power supplied from the alternating current power supply 1 to the magnet switch is interrupted, and thus the supply of electric power to the servomotor is interrupted. Therefore, if the operator releases the deadman switch DM2 during the teaching procedure, the deadman switch DM2 is then turned OFF as shown by an arrow A3, whereby the operation of the robot is immediately stopped.
The traditional deadman switch is a 2-position contact type in which the switch stays ON as long as the movable part of the switch is depressed by a finger, and is turned OFF when the pressure on the movable part is released.
Accordingly, even if a robot at rest should accidentally start and approach the operator during a teaching session, the robot can be immediately stops if the operator stays cool and releases the deadman switch DM2 on the teaching control device.
Such a robot emergency stop situation, however, is an unexpected and unwelcome occurrence for the operator, and thus the operator tends to become habitually nervous and jumpy. Therefore, even if the operator knows that the robot can be stopped by releasing the deadman switch during regular teaching session, in an extreme pressure situation the operator may take an unintended action. For example, the operator may be panic-striken by a sudden approach of the robot and may try to gain protection from the approaching robot by unconsciously holding on tightly to the teaching control device.
Obviously, if the operator holds the teaching control device tightly, the robot cannot be stopped by the traditional deadman switch; as it stays ON as long as the movable part thereof is depressed by a finger and will turn OFF only when the pressure on the movable part is released.
Such a problem would not occur even if the panicking operator unconsciously holds the teaching control device tightly, if the deadman switch were designed to stay ON as long as the movable part is not under pressure and to turn OFF when the movable part is depressed by a finger. This design, however, may also lead to a hazardous condition if the operator, panic-striken by an approaching robot, should drop or throw away the teaching control device.